1. Field of the Invention
The present invention relates to an exposure apparatus configured to expose a substrate to light and a device manufacturing method using the exposure apparatus.
2. Description of the Related Art
A reduction projection exposure apparatus is mainly used in manufacturing an electronic device such as a semiconductor device. The reduction projection exposure apparatus transfers a circuit pattern of a reticle (a photomask with the circuit pattern) onto an exposure target wafer (substrate) to which a photoresist (a photosensitive material) is applied, while reducing the circuit pattern. Currently, a scanning exposure apparatus (scanner) is in the mainstream of the exposure apparatus.
The scanning exposure apparatus is an improvement of a stationary exposure apparatus (stepper). In the stepper, a reticle pattern is exposed with a single shot exposure. Since the stepper uses a single shot exposure, local focus and tilt correction in a shot area has been difficult.
On the other hand, the scanning exposure apparatus transfers a circuit pattern of a whole reticle onto a wafer by scanning the reticle using a light which passes through a slit. The scanning exposure apparatus is capable of following a change in a local surface shape in a shot area since the scanning exposure uses light that passes through the slit and a distortion in a scanning direction of a projection optical system can be averaged. In a focus/leveling drive of a wafer stage during the scanning exposure, a pre-reading sensor and a sensor detecting a wafer shape in a slit area of a shot to be exposed are used to determine a focus/tilt amount. The sensors are included in a scanning focus measuring apparatus.
With these sensors, the wafer stage is controlled to preliminarily follow a target position on a best focus image plane obtained from the scanning measurement result before a photoresist is exposed. The above is performed before a step of an exposure target shot passes through an area irradiated with an exposure light which is limited by an exposure slit.
However, according to a scanning exposure method using the pre-reading sensor, a position of the pre-reading sensor of a scan focus measuring apparatus is determined in advance by a hardware design of the apparatus. As a result, if a scanning exposure speed is too fast or if a deformation of the wafer shape exceeds a certain amount due to a process factor or the like pre-reading driving may not be finished in time and may cause a negative effect on focus accuracy or synchronization accuracy. Japanese Patent Application Laid-Open No. 2001-332471 discusses an exposure apparatus which is capable of minimizing an effect of a defective shot on shots in the vicinity thereof and of reducing a number of defective shots. The exposure apparatus is capable of reducing inappropriate operations made by an operator and is capable of detecting a wafer chuck contamination in the exposure process.
In recent years, a layout and an invalid area are reexamined for improvement of yield per wafer, which contributes to improvement of total productivity. In order to improve the total productivity, it is useful to transfer more mask patterns onto a single wafer. Accordingly, a shot is arranged not only to fit all angles of view into a wafer but also to use an outer periphery on the wafer edge. A number of processed wafers per unit time tends to increase year by year. This is achieved by setting as fast a scanning speed as is feasible.
However, with respect to a processed wafer which repeatedly goes through the processes of photoresist application, exposure, and development, it has been difficult to assure highly-accurate focus/leveling drive of a shot near the edge since the wafer shape on the outer periphery is not always favorable. The wafer shape is known to be deformed by heat treatments performed during processing other than the exposure process, for example, a restoration treatment for crystal destruction caused by ion implantation and metal processing of a surface layer of the wafer and metal processing after wiring. Further, the wafer is known to be deformed by heat treatments performed during processing, such as, removal of solvent which hinders pattern formation after a photoresist layer is applied on the surface of the wafer.
Most of the deformed wafers have their outer periphery significantly warped upward or downward. Where an amount of warp exceeds a certain level, shots at the outer periphery may be tilted several hundreds of μrad and deformed in a few μm order. Further, the peripheral area of processed wafers can be locally deformed. At local points of the peripheral area, the surface often has sharp projections and depressions, and the reduction in flatness is unfavorable.
As to such wafers, focus accuracy of the peripheral area is a matter of concern. Focus accuracy problems occur when, for example, a shot is located partially beyond a valid area and thus cannot be measured by a focus detection system or when an unfavorable reduction in flatness at the peripheral area causes incorrect focus/tilt measurement value. When these problems occur, a focus of the exposure portion within a shot is adjusted not to a point where it should be focused (the best focus image plane) but to a distant point. Therefore, defocusing which causes defective exposure shot occurs in the peripheral area of the wafer.
Furthermore, in a peripheral area of a wafer, a great difference in focus/tilt planes may be seen between adjacent shots. A large amount of focus/leveling drive becomes necessary for such shots. In such cases, a wafer stage may be unable to follow a position of the best focus image plane measured by the pre-reading sensor of the focus detector from a position where scanning of the shot is started, and as a result, exposure of the shot may be started even if the shot area is defocused. A similar incorrect drive occurs if a focus invalid area which is a measurement inhibition area is not properly set or if an abnormal value is generated by dust on the wafer.